1. Field of the Invention
The present invention relates to an improved cathode having lateral extensions of "glove fingers" type, and, more especially, to such cathode for an electrolysis cell, the electrical conductivity of which, in particular, is markedly improved.
2. Description of the Prior Art
Cathodes "in the form of traversing or non-traversing glove fingers", depending upon whether they extend from one side of the cell to the other, or leave a passage between their respective inner ends for the circulation of the electrolyte, have been used for several decades for the electrolysis of alkali metal chlorides, and in particular for the preparation of chlorine and caustic soda. Such cathodes are charged with electric current via the vertical sidewalls of the cell and via a cathode band formed by a plate folded in the shape of a horseshoe, or by several plates of highly conductive metal, applied to the outside of one or more of said cell walls. This electric current can be homogeneously distributed inside the cells, and in particular over the cathode surfaces active in the electrolysis, opposite the anodes, by means of members having a minimum resistance.
It will also be appreciated that, in the chlorine industry, the term "cathode" frequently connotes not only the internal portion of the cell playing an active role in the discharge of the positive ions, but also the vertical sidewalls or enclosure of the cell and the cathode band, namely, the entire assembly which directs the current from the conductor connected to the negative source of current, generally via other cells, to the electrolyte. The term "cathode assembly" will be employed hereafter to connote the "cathode" in this broad sense, which therefore includes a portion inside the cell, which will be referred to as the "internal cathode" and is itself formed by a peripheral chamber in contact with the walls, with tubular extensions of rectangular cross-section, or fingers, in communication with the said chamber, and an external portion formed by the vertical sidewalls or enclosure of the cell and by the cathode band.
The internal cathode, which consists of perforated metal or, most frequently, of iron or steel mesh, generally supports the diaphragm, which is deposited by filtration, on the perforated metal serving as the filter, of a slurry containing the solid material of this separator, by means of a partial vacuum created inside the cathode. Asbestos fibers, which long have constituted the essential element of this material, are now augmented or even replaced by fluorinated resins, which require, after deposition, baking at a temperature which can be on the order of 400.degree. C., in order to consolidate this diaphragm by sintering. The pressure which is exerted on the internal cathodes and the tensions generated by the heat treatment, in the case where these electrodes have a large surface area, frequently result in deformations and detract from their inherent flatness and from the parallelism of the active surfaces among themselves and with respect to the anodes.
To overcome these various noted disadvantages, it was necessary to arrange strengthening or reinforcing elements in the form of corrugated steel plates, inside the cathode fingers; these plates, on which the internal cathode is constructed, prevent the fingers from being crushed during the deposition of the diaphragm. An assembly of this type does not completely overcome the difficulties referred to above; furthermore, the contacts between these elements and the perforated walls of the internal cathode are point contacts and are frequently made via oxidized surfaces. Thus, they play virtually no role in the conduction of the currents, all the more so because the electrical and mechanical connections between the wall of the cell and the active part of the cathode are made essentially via the lower and upper perforated walls of the peripheral chamber.
French Pat. No. 2,287,527 proposes the use of spacers in the form of rectilinear plates with notched longitudinal edges. More precisely, the said patent recommends the use of spacers, the teeth of which have a cross-section in the shape of a rectangle, the sides of which are in one case larger and in another case smaller than the diameter of the perforated plates, the pitch of the teeth being different from that of the perforations in the perforated plates which form the box structure of the cathode elements (page 16, line 14, to page 17, line 2, of the above-mentioned patent).